This invention relates to a system for the humidification of a polymer electrolyte membrane fuel cell (PEMFC), a fuel cell containing such a system, and a method of modifying a fuel cell to include such a system.
A PEMFC has an air or oxygen facing surface and a fuel facing surface. The electrolyte used comprises a thin (10-200 xcexcm) proton conducting polymer membrane, often based on perfluorocarbonsulphonic acid. A catalyst layer is applied to either side of the membrane, followed on the air or oxygen facing side by a gas diffusion layer, and on the fuel facing side by a gas diffusion layer. The gas diffusion layer is commonly a carbon cloth or paper. Often the gas diffusion layer is treated to ensure that the surface in contact with the catalyst layer is hydrophobic, ensuring that water is removed from the immediate vicinity of the catalyst layer. Examples of gas diffusion layer include xe2x80x98Toray(trademark) carbon paperxe2x80x99 made by Etek (USA), or xe2x80x98Carbel(trademark)xe2x80x99 manufactured by W L Gore (USA). Finally an electrically conductive material is added to the air or oxygen side to form a cathode and an electrically conductive layer is added to the fuel side as an anode. In use, a fuel, most commonly hydrogen gas, is supplied to the anode and oxygen, either pure or more usually from the surrounding air, is supplied to the cathode. The cell reaction can be represented as follows:
at the anode: H2xe2x86x922H++2exe2x88x92,
at the cathode: xc2xdO2+2H++2exe2x88x92xe2x86x92H2O.
The by-product of the cell reaction is water. The efficiency of a PEMFC is strongly dependent on the conductivity of the polymer membrane, which is in turn dependent on the amount of water retained within the membrane.
Polymer electrolyte membrane fuel cells have a polarisation curve that gives rise to maximum power at approximately 0.5V per cell. It is advantageous to run a fuel cell system at maximum power, however resistive losses cause the temperature of the fuel cell to increase. This increase in temperature drives water from the polymer electrolyte membrane causing it to dry out. As the membrane dries out its resistance increases. This leads to a further increase in temperature, which in turn drives more water from the membrane. Efficient water management is therefore essential to enable a fuel cell to be operated continuously at maximum power.
The problem of dehydration of polymer electrolyte membranes has been addressed in several ways. EP 0980106 describes a system whereby water is supplied as a liquid to the cathode and thence to the electrolyte membrane. This provides the necessary humidification, but requires a separate water supply and pumping system. This adds complexity, weight and bulk to a polymer electrolyte fuel cell power plant.
An alternative method is to pass the fuel gas, usually hydrogen, through water before supplying it to the fuel cell. Again, this requires external equipment and a supply of water.
A still further method is described in U.S. Pat. No. 5,503,944. This uses a closed water circulation loop that provides the necessary water to the electrolyte and additionally functions as a cooling system. This also requires additional equipment and, in common with the other approaches described above, circumvents the problem of dehydration of the electrolyte membrane by providing an additional water supply.
In accordance with the present invention, there is provided a polymer electrolyte membrane fuel cell provided with a humidification system comprising at least a first layer and a second layer, wherein the first layer comprises an air permeable, absorbent layer disposed adjacent or in close proximity to a first surface of a polymer electrolyte membrane fuel cell, and wherein the second layer comprises a non-absorbent material disposed adjacent or in close proximity to the first layer, the second layer having through openings therein to allow passage of air through the second and first layers to the fuel cell interior.
The present invention retains water near to the first surface of the PEMFC and therefore the membrane of the PEMFC, whilst still allowing sufficient air to reach the cathode. This maintains the humidity of the membrane, and hence its conductivity. Maximum power can thus be drawn from the fuel cell for extended periods. Unlike the prior art approaches described above, the present invention addresses the problem of dehydration of the polymer electrolyte membrane by efficiently managing the water produced during operation of the fuel cell, not by providing an additional water supply. Water storage and associated pumping and supply means are therefore not required, leading to savings in size and weight and improved robustness.
The first layer will usually be in direct contact with the fuel cell outer surface and will not be electrically conductive, although in some arrangements an intermediate layer may be present.
Preferably, the first surface of the polymer electrolyte fuel cell comprises an air or oxygen facing surface. This may be a cathode or a cathode current collector.
It may be that the cathode or cathode current collector is provided with a protective surface layer or coating. This may be for example, a layer of plastic that is used to prevent damage to the cathode. In this case, the first layer may be substantially co-incident with this protective coating.
The non-absorbent material may be in the form of a substantially solid structure forming an impervious barrier or shell, except for the through openings that are provided to permit inflow of air. The through openings may comprise preformed passageways or holes or may be subsequently provided as perforations. In any case, they extend from one surface of the second layer to its opposite surface to allow air to pass through the second layer. The openings (eg passageways or perforations) in the second layer are most conveniently circular as this shape is simple to produce, for example by drilling. Clearly, any other shaped passageways or perforations, such as slots or squares may equally be used. Furthermore, the passageways or perforations in any individual second layer need not all be of a similar size.
The preferred size and number of through openings in the second layer is such that a balance is reached between an adequate supply of air (oxygen) to the cathode and efficient retention of water in the vicinity of the polymer electrolyte membrane. The open surface area in the second layer would usually not exceed 10% and for most applications will not be more than 5%.
Preferably, the non-absorbent material comprises a rigid material.
Preferably, the non-absorbent material comprises stainless steel, nickel, titanium or other metal.
Alternatively, the non-absorbent material may comprise a non-metallic material such as PTFE or related polymer, a plastic material, or a composite material such as Kevlar(trademark).
The first and second layers will usually be in contact with one another and the fuel cell outer surface, and may be held or fastened tightly in contact with each other. Preferably, the non-absorbent material of the second layer is in the form of a sheet with a surface having raised features. These features may comprise a series of interconnected voids or channels forming ridges or corrugations, or may constitute a series of peaks and troughs such as those found on an xe2x80x98egg boxxe2x80x99. This creates voids between the first and second layer where air may circulate. On such a ridged second layer the ridges should preferably not exceed 5 mm in height, and may be curved or angular in shape.
Preferably, the through openings in the second layer are not in direct contact with the first layer.
The second layer may be in the form of a cover for the fuel cell, optionally with attachment means, which may be secured tightly on or around the fuel cell.
The air permeable, absorbent first layer stores water in a region in close proximity to the membrane. The layer will normally comprise a porous material, which may comprise a fibrous woven or non-woven material. The first layer ideally comprises a hydrophilic material. Usually, it will be formed of a sheet or plurality of sheets, at least one of which is of a water absorbent nature, and may comprise a cloth, with cotton, other natural fibres or absorbent synthetic fibres being particularly suitable. Other examples could include paper or other sheets that have water wicking and, retaining properties.
The porosity of a porous material will affect the performance of the fuel cell. A highly porous material helps air to permeate to the cathode of the fuel cell, but also increases the rate of dehumidification. Conversely, a low porosity material is advantageous with respect to preventing the membrane from drying out, but has a tendency to starve the cathode of air (oxygen).
Advantageously, the surface (or a surface layer region) of the first layer adjacent to the fuel cell is hydrophobic. For example, the layer may be treated so that it is water repellent on the side adjacent to the fuel cell. This assists in retaining water within the gas flow path, ensuring humidification. Several methods can be used to make the surface or surface layer region water repellent, such as plasma coating or spraying with-a perfluorinated polymer, which may include polytetrafluoroethylene (PTFE), other polyfluoroalkyls or polyfluorosilanes, or other polymers such as polyvinyidenefluoride (PVDF), halo amino and other substituted triazine polymers. Particularly suitable are the superhydrophobic fluoroethane coatings manufactured by the Cytonix Corporation, Fluorothane(trademark) and FluoroSyl(trademark), also Tullanox(trademark) manufactured by Tulco Inc, and plasma coatings ParaLast(trademark) and Parylene(trademark).
Alternatively, a separate hydrophobic, air permeable layer could be interposed between the fuel cell and the first layer to provide the water repellent function at the fuel cell surface, so that treatment of the first layer is not required.
Preferably, the system further comprises an air permeable, absorbent layer substantially coincident with a second surface of a polymer electrolyte membrane fuel cell.
Preferably, the second surface of the polymer electrolyte membrane fuel cell comprises a fuel facing surface. This may be a fuel flow plate, a former, an anode or an anode current collector. Moisture which is retained in the first layer is carried back to the fuel cell in the fuel stream. This provides additional humidification to the membrane of the fuel cell, further improving performance.
In a further aspect, the present invention provides a humidification system for a polymer electrolyte membrane fuel cell comprising an air permeable, absorbent first layer, a second layer of a non-absorbent material, which layer has through openings therein, and attachment means for securing the first and second layers in that respective order to a first surface of a polymer electrolyte membrane fuel cell. The humidification system may comprise a kit for modifying a planar fuel cell or a tubular fuel cell.
In summary, the impervious second layer of the humidification system minimises water losses by convection and transpiration, enabling the absorbent, preferably hydrophilic, fraction of the first layer to retain water therein, while the hydrophobic surface layer or surface region, if provided in contact with the fuel cell body, ensures that gas access to the fuel cell is maintained.
The present invention further provides a system for the humidification of polymer electrolyte membrane fuel cells comprising a first layer and a second layer; wherein the first layer comprises a porous material substantially coincident with a first surface of a polymer electrolyte membrane fuel cell; wherein the second layer comprises a non-absorbent material substantially coincident with the first layer; and wherein perforations in the second layer allow air to flow through the first and second layers to a cathode of the polymer electrolyte fuel cell.